America is aging. According to the US Census Bureau, about 14 percent of the population is 65 or older. But, as Baby Boomers age, this number will rise. By 2030, 1 in 5 people will be 65 or older.

The demographic shift has implications for a number of areas, including healthcare. As people age, they are more likely to experience various ailments and disabilities. And, this usually means their doctors prescribe more medicine for them, too.

Increasingly, however, doctors in South Dakota are prescribing something different: physical activity. According to National Geographic, doctors are using a notepad with an RX symbol on the top left corner to instruct their patients to take one free day at any South Dakota state park or recreation area.

The program is supported by the South Dakota Game, Fish, and Parks, in collaboration with the state’s Department of Health. It was pilot tested in 2015 and is now implemented state-wide. South Dakota is not alone, as doctors in states from New Mexico to California, among others, have implemented similar approaches.

But does it work? According to the American College of Sports Medicine, the answer is a resounding Yes. Regular exercise cuts the risks of certain cancers, the incidence of Type II diabetes, cardiovascular diseases, and even Alzheimer’s. So confident in the findings, the organization has coined the term ‘exercise is medicine.’

There are other benefits, too. According to a recent study, seniors regularly pay over $10,000 in annual medical costs. As most forms of physical activity are free, prescribing exercise is a cost effective alternative.